Cellulosic fibrous web structures are used in a variety of common consumer products. For example, cellulosic fibrous structures are used as paper towels, toilet tissue, facial tissue, napkins and the like, commonly known as nonwoven. The large demand for these various paper products has created a demand for improved versions of the products and the methods of their manufacture.
Although single ply structures are used, multiple ply cellulosic fibrous structures are well known for use in the production of the consumer products. Such products are cellulosic fibrous structures having more than one ply laid over each other. These layers or plies are typically bonded to prevent delamination to maintain structural integrity when the final multilayer nonwoven is in use.
Bonding of the individual cellulosic fibrous webs into multi-ply structures is often achieved by embossing the structures on converting equipment without applying an adhesive. In a typical embossing process, two or more plies are fed through a nip formed between juxtaposed axially parallel embossing rolls. Embossment knobs on these rolls force each ply into engagement and contacting relationship with the opposing ply. The compressed regions of the plies have the benefits of producing an aesthetically pleasing pattern as well as providing a means for joining and maintaining the plies in face-to-face contact.
Controlling ply or web bonding is often difficult to achieve. Inadequate, excessive or inconsistent ply bonding can jam complex, high-speed machinery; generate high waste, and causing reduction in production rate and poor quality. The strength of bonding by embossing (without adhesive) may vary depending, among other things, on water content or dryness of fibrous webs, and on ambient air humidity. While bonding by embossing sometimes can be improved by increasing the pressure on the embossing rolls, such pressure can wear out the embossing rolls more quickly, particularly the female roll, which is usually a softer roll made of composite material onto which a pattern is impressed by the opposing, male, engraved metal roll. In addition, the journals and bearings of both embossing rolls can also wear out prematurely if subjected to increased pressure over a prolonged period.
In a different process, fibrous plies may be bonded using a chemical adhesive rather than by embossing. For example, U.S. Pat. No. 5,466,318 discloses a process for laminating webs using a water-based adhesive. Adhesive bonding is sometimes used to avoid some of the problems of bonding through embossing, but adhesives also cause problems, such as “through bonding” or “blocking”, in which adjacent laminates bond to one another, and, prevent unwinding of the laminate product from its roll. Non-uniformity of bonding also causes problems, such as wrinkling of the tissue and bad printing.
U.S. Pat. Nos. 6,602,577, 6,635,134, and 6,649,025 disclose multi-ply paper structures in which an adhesive composition is used to bond the plies together. A variety of adhesive compositions are disclosed, including polyvinyl alcohol-based adhesives.
U.S. Pat. No. 3,135,648 disclosed that polyvinyl alcohol adhesives with high wet tack may be prepared by adding a water-soluble-boron compound to the copolymer and by adding an acid to lower pH.